Can PCAAD 7.0
be used to calculate mutual coupling between microstrip antennas?

The short answer
to each of these questions is: no.

The purpose of
PCAAD 7.0 is to provide an easy-to-use, fast, and versatile first-cut
solution
to a number of common antenna design and analysis problems. PCAAD is
also
offered as a relatively inexpensive software tool that has a very short
learning
curve. For these reasons, PCAAD 7.0 is not suited to the intensive sort
of
numerical analysis that is required for problems of the type listed
above. For
these problems, much more sophisticated software is needed. Packages
such as
Ansoft's Ensemble and HFSS, Zeland's IE3D, Microwave
Office, and similar products are well-suited to working with very
general
2D, 2.5D, and 3D geometries. Of course, you should realize that these
software
products typically range in cost from $25,000 to $50,000,
while PCAAD 7.0
lists for $549. So it is not reasonable to expect PCAAD 7.0 to have the
same
capabilities as these more expensive products.

What
is the useful frequency range for PCAAD 7.0 ?

PCAAD 7.0 has been used for antenna analysis and design problems ranging from
low RF
frequencies (20 MHz) to high millimeter wave frequencies (94 GHz).
Antennas
generally scale with frequency, so a solution that works for one
frequency in
principle will also work at lower (or higher) frequencies as the
antenna
dimensions are scaled up (or down). There is no inherent frequency
limit for the
solutions implemented in PCAAD, but of course there are practical
frequency
limits for specific types of antennas.